GREEN ONION PRODUCTION
IN CALIFORNIA
RICHARD SMITH, UCCE Vegetable Crop Advisor, Monterey County;
MICHAEL CAHN, UCCE Irrigation and Water Resources Advisor,
Monterey County; MARITA CANTWELL, Extension Vegetable Crops
Specialist, Department of Plant Sciences, University of California, Davis;
STEVEN KOIKE, UCCE Plant Pathology Farm Advisor, Monterey County;
ERIC NATWICK, UCCE Farm Advisor, Imperial County; ETAFERAHU
TAKELE, UCCE Area Farm Advisor, Southern California Counties
PRODUCTION AREAS AND SEASONS
Green onions (Allium cepa L.) are produced mainly
in Monterey, Riverside, and Ventura Counties, but
there is also small-scale production in other parts of
the state. Harvest and bunching of this crop is labor
intensive, and over the last several years substantial
production has moved to Mexico, where the crop is
less expensive to produce. Green onions are planted
from spring through fall for nearly year-round
harvest.
CLIMATIC REQUIREMENTS
Onions are cool-season biennial plants (requiring two
seasons to complete the cycle from seed to seed) that
are commercially grown as an annual crop. Bulbing
and growth are highly sensitive to day length. Each
variety has a critical day length at which bulbing is
initiated, regardless of size. As days lengthen in the
spring and summer, these critical day lengths are
reached.
Growth is also dependent on temperature. The
minimum for emergence is higher than for most
other cool-season vegetables, at 55ºF (12.8ºC) for 70
percent emergence in up to 2 weeks. In addition,
early growth rate is slow compared with other cool-
season crops. Optimal leaf growth rate occurs at 68º
to 77ºF (20º to 25ºC). However, the total plant growth
rate depends on the amount of light intercepted.
Combinations of factors, including variety, stem size,
temperature, and duration of temperature, determine
bolting susceptibility. Onions are stimulated to bolt at
temperatures of 45º to 50ºF (7.2º to 10ºC).
VARIETIES
Few public onion breeding programs exist in the
United States (none in California), but private
seed companies are involved in onion variety
development. The emphasis of variety development is
on hybrid types.
Green onion varieties are generally classified
according to day length (short- and long-day types).
Long-day white varieties of Sweet Spanish or
Southport White Globe are grown as green onions in
areas with short days. Short-day varieties bulb too
easily to be used for green onion production. Many
hybrids have been developed by crossing Allium
cepa with A. fistulosum, the nonbulbing Japanese
bunching onion. These hybrids are most commonly
grown in spring and summer in the Salinas Valley,
and occasionally during winter in the southern desert
regions and in Baja California, Mexico. Green onion
production in areas with long days or during the
summer months is most successful with the use of
varieties with Japanese bunching onion parentage.
White Sweet Spanish and Southport White Globe
types are also sometimes grown in intermediate- and
long-day growing regions.
PLANTING
Green onions are planted in dense stands on beds
40 or 80 inches (1 or 2 meters) wide. The crop is
seeded in dense plantings with 18 to 20 seed lines on
80-inch beds. Seed is planted approximately 0.5 inch
(12.5 mm) deep; this shallow planting requires a soil
surface that is well prepared and that is kept moist
through germination. Onion seed is susceptible to loss
Vegetable
Production Series
vric.ucdavis.edu
UC Vegetable Research
& Information Center
University of California
Agriculture and Natural Resources
http://anrcatalog.ucdavis.edu • Publication 7243
GREEN ONION ACREAGE AND VALUE
Year Acreage
Average yield
(tons/acre)
Gross
value/acre
2009 1,504 16.05 $18,589
2008 1,633 13.04 $13,651
2007 2,045 14.64 $15,143
2006 1,495 13.02 $13,312
Source: County agriculture commissioners’ annual report data,
Monterey and Riverside Counties.
http://vric.ucdavis.edu
http://anrcatalog.ucdavis.edu
of vigor from high temperature and humidity. The
minimum level of germination is 85 percent for onion
seed accepted and packaged by seed companies
in California. Seed size may range from 100,000 to
130,000 seed per pound (220,000 to 286,000 seed/kg).
Plantings of green onions require 12 to 18 pounds
of seed per acre (13 to 20 kg/ha) or 1.2 to 2.3 million
seed/acre (3.0 to 5.7 million seed/ha).
SOILS
Green onions will grow in a wide range of soil
types. They grow best in well-drained soils such as
sandy loam, loam, and clay loams. Green onions are
shallow-rooted and need a friable soil that retains
moisture. Sandy soils require frequent irrigation but
are less likely to adhere to roots at harvest than are
clay soils. Plantings in clay soils are often avoided
for early-spring crops due to the extra time needed
for the soil to dry after rain. Seed germination and
seedling establishment require a seedbed that is
uniform, clod-free, firm, and several inches deep.
Compared with planting on flat or small ridges,
raised beds provide better drainage and an area for
salt accumulation away from the root zone.
Onions are sensitive to salinity, primarily at
the stages of germination and emergence. Once
plants are established, higher levels of salinity are
tolerated. Yield reductions may occur in soils with
an electrical conductivity greater than 1.2 dS/m
(mmho/cm) or that are irrigated with water with an
electrical conductivity greater than 0.8 dS/m. A 50
percent yield reduction may occur at soil electrical
conductivity levels of 4 to 5 dS/m. Onions are
sensitive to boron in concentrations greater than 0.5
mg/L in irrigation water.
IRRIGATION
Overhead sprinklers are typically used for green
onion production. Onion seed must not dry out
during germination, and the soil surface should
not be allowed to crust during seedling emergence,
which may last 10 to 20 days following the initial
irrigation. After emergence, irrigation with overhead
sprinklers is continued until harvest. Drip irrigation is
uncommon in green onion production because of the
close spacing of rows.
Green onions require frequent and uniform
irrigation. Because 90 percent of the roots are in
the upper 8 inches (20 cm) of the soil, little water is
extracted from the soil at a depth of more than 18
inches (46 cm). Moisture is required near the surface
of the soil to stimulate new root growth because
onion roots are mostly nonbranching, and all roots
originate at the stem, or basal plate, of the plant. Mild
water stress can reduce yield or cause uneven growth
patterns in the field.
The amount and frequency of irrigation
depends on the soil type, weather conditions, and
development stage of the crop. After emergence
when plants are small, water needs are low, so the
irrigation interval with overhead sprinklers may
range from 7 to 10 days. As plants increase in size,
water use increases, and irrigations may become
more frequent or longer to maintain uniform soil
moisture in the beds. A green onion crop typically
uses 10 to 15 inches (25 to 38 cm) of water to meet
evapotranspiration requirements. Most growers
apply 12 to 20 inches (36 to 51 cm) to achieve
maximum yields. Water is cutoff 3 to 5 days before
harvest, depending on the soil type. Supplemental
irrigation may be needed to maintain the remaining
crop during harvest, which may last 5 to 7 days.
The combination of soil moisture monitoring and
weather-based irrigation scheduling can be used
to determine the water needs of green onions. For
optimal plant growth on most soil types, irrigate to
maintain the soil water tension less than 30 cbars
(kPa) in the upper 8 inches (20 cm) of the soil. Water
use is highest when the crop reaches full canopy
cover. The water extraction of onions can be
estimated using reference evapotranspiration data
adjusted with a crop coefficient that is closely related
to the percentage of ground covered by the leaf
canopy. At a maximum canopy cover of 85 percent,
the crop coefficient is nearly 1.0. Because evaporation
represents a majority of the water loss during the early
stages of growth, a crop coefficient between 0.3 and 0.5
should be used for overhead sprinklers, depending on
the irrigation frequency, until the leaves shade more
than 30 percent of the ground. Maximum yields are
reached with applied water from 100 to 150 percent
of crop ET. The California Irrigation Management
Information System (CIMIS), coordinated by the
California Department of Water Resources, provides
daily estimates of reference evapotranspiration for
most production regions of California at their Web
site, http://wwwcimis.water.ca.gov.
FERTILIZATION
Because onions are shallow rooted and often
planted in cool soils, they are responsive to
fertilization. Soil analyses are the best indicators for
phosphorus (P), potassium (K), and micronutrient
needs. Soi ls with bicarbonate extractable
phosphorus less than 10 ppm may require up to
200 pounds of P2O5 per acre (224 kg/ha), but soils
with phosphorus levels greater than 30 ppm may
need as little as 50 pounds of P2O5 per acre (56 kg/
ha). With adequate preplant application, in-season
phosphorus application is seldom warranted.
Soils with less than 100 ppm ammonium acetate—
extractable potassium may require up to 150
2 • Green Onion Production in California
http://wwwcimis.water.ca.gov
pounds of K2O per acre (168 kg/ha) to ensure
adequate potassium supply. However, soils in
with more than 150 ppm ammonium acetate–
exchangeable potassium are unlikely to respond to
potassium fertilization. Most California soils have
adequate availability of all micronutrients; where
micronutrient deficiency occurs, zinc is often the
most limiting nutrient.
Soil nitrate levels and cropping history are the best
indicators for nitrogen (N) needs. Typically, no more
than one-third of the nitrogen should be applied at
planting, one-third at early-season (2 to 3 leaf stage),
and one-third at midseason. Total supplemental
nitrogen needs may vary from 100 to 200 pounds of
nitrogen per acre (112 to 224 kg/ha). Marketability
requires that leaves be dark green and without
yellowed or necrotic leaf tips.
INTEGRATED PEST MANAGEMENT
UC IPM Pest Management Guidelines for onions
have been updated (including photographs) and are
available for weed, insect, disease, and nematode
pests; see the UC IPM Web site, http://www.ipm.
ucdavis.edu. Sanitation, crop rotation, resistant
varieties, and frequent monitoring are essential
for prevention and control of the numerous pests
afflicting onions.
Weed management. Onions compete poorly with
weeds because they initially grow slowly, and the
crop canopy does not provide complete ground
cover. Dense plantings of green onions do not allow
for effective use of cultivation. Weed control in
conventionally produced green onions consists of
the use of a preemergence herbicide followed by
a postemergence herbicide. Topical applications of
liquid fertilizers at the second true leaf stage can kill
many broadleaf weeds. Organic producers rely on
cultural practices such as rotating into fields with low
weed pressure as well as using preirrigation followed
by shallow cultivation to kill an initial flush of weeds.
Under both production systems, hand weeding is
typically required to control weeds later in the growth
cycle. However, the cost of hand weeding can vary
widely depending on the effectiveness of early season
weed control efforts.
Insect identification and management. Thrips
(western flower and onion) and maggots (seed corn
and onion) are the most frequent serious insect
problems in onions, but bulb mites, leafminers, and
armyworms are occasionally serious pests.
Bulb mite is most damaging when plant growth
is slowed by cool, wet weather. The mites cut off
the radical of germinating seed before the plant
becomes established or penetrate the outer layer of
tissue and allow rot-causing organisms enter the
bulbs. Decaying cole crops or other crop residues
cause a rapid buildup of mites. The best management
option is to allow the organic matter to decompose
completely, which causes the mite population to
crash.
Maggots, similar to bulb mites, are favored by
cool, moist soils with high levels of nondecomposed
organic matter. Seed corn maggot larvae attack
germinating seedlings, feeding on the developing
roots and epicotyl. Onion maggots also damage
seedlings but continue feeding on the expanding
bulb. Allow complete decomposition of organic
matter prior to planting or transplanting; if this is not
possible, a soil applied insecticide may be used.
Thrips can reduce yield through unacceptable
cosmetic damage to the leaves. Sprinkler irrigation
can help reduce thrips numbers, but for high
populations foliar insecticide applications are
required to prevent injury.
Disease identification and management. Pink
root (Phoma terrestris), white rot (Sclerotium cepivorum)
and Fusarium basal rot (F. oxysporum f. sp. cepae) are
the most common and important diseases in green
onions. Bacterial rots (Pseudomonas and Erwinia spp.),
downy mildew (Peronospora destructor), purple blotch
(Stemphylium vesicarium, Alternaria porri), black mold
(Aspergillus niger), neck rot (Botrytis allii), blue mold
(Penicillium hirsutum), and smut (Urocystis cepulae) are
occasionally observed.
Nematode identification and management. Stem
and bulb nematode (Ditylenchus dipsaci) and root-knot
nematodes (Meloidogyne spp.) can be problems in
California onion production, but these do not occur
frequently.
HARVESTING AND HANDLING
Fresh green onions require hand-harvesting. The
most common system used involves undercutting the
onions, pulling them immediately, gathering them
by hand into bunches of five to seven plants tied
together with rubber bands, and placing them into
20-pound cartons. Other containers used include
11-, 13-, and 28-pound cartons; these containers are
usually for export or are used for imported green
onions.
POSTHARVEST HANDLING AND
STORAGE
Green onions are highly perishable. Shelf life is only 7
to 10 days. To minimize loss, green onions are stored
at 32ºF (0ºC) and 95 to 100 percent relative humidity.
If green onions are improperly stored, the leaves will
yellow and decay. To keep the moisture content high,
top ice can be used, or the onions can be covered with
plastic film.
3 • Green Onion Production in California
MARKETING
California leads the United States in green onion
production. Shipments are made from California
year-round, but supplies peak from July to October.
The winter and spring supply of green onions is
dominated by imports from Mexico, whose
production schedule is very similar to that in the
southern California desert regions. South Carolina
and Ohio are other states with sizeable production
of domestically grown green onions.
4 • Green Onion Production in California
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GREEN ONION PRODUCTION IN CALIFORNIA
Production Areas and Seasons
Climatic Requirements
Green Onion Acreage and Value
Varieties
Planting
Soils
Irrigation
Fertilization
Integrated Pest Management
Harvesting and Handling
Postharvest Handling and Storage
Marketing
For Further Information
